Aspen Regeneration in South-Central
Colorado, San Isabel National Forest
Tim Benedict1
Abstract—The potential for aspen regeneration in conifer stands has been underestimated on the Salida Ranger District. Harvest of mature aspen stands on the Salida and
San Carlos Districts encouraged regeneration. Following harvest, the Douglas-fir and
some Engelmann spruce stands in the Arkansas Hills area regenerated primarily to
aspen. Disturbance through aspen harvest, prescribed fire, wildfire, and other approaches is critical to aspen sustainability and health for the San Isabel National Forest.
Disturbance must take place to regenerate the aspen component and reduce the
conifer invasion.
Introduction
Q
uaking aspen (Populus tremuloides) covers about 80,450 acres of the San
Isabel National Forest’s Salida and San Carlos Ranger Districts in southcentral Colorado (figure 1). Although aspen is present in many conifer sites
throughout the San Isabel National Forest, aspen occupies only about 10% of
the two ranger districts. The potential for aspen regeneration in conifer stands
(i.e., Douglas-fir) has been underestimated on the Salida Ranger District.
Disturbance through aspen harvest, prescribed fire, wildfire, and other approaches is critical to aspen sustainability and health for the San Isabel National
Forest. About 13,320 acres of aspen are suitable sites for harvest (figure 2).
In the past, the public and small purchasers on the San Isabel have primarily
harvested aspen for fuelwood. Only recently has there been an increased demand
for aspen fiber. This is due to a decrease in local supply that results in purchasers
having to haul longer distances to meet their current demand. The majority of
aspen could be utilized by mills found in Delta and Olathe, Colorado. These mills
produce aspen flakeboard and panels.
Aspen is highly valued for its scenic beauty. In south-central Colorado, it is
found at elevations ranging from 8,200 to over 10,000 feet (Powell 1988). In
the fall, usually mid to late September, many visitors come to view the vibrant
colors of yellow, gold, orange, and red. People seek out the aspen to take
pictures, stopping along the roadside to view it. Some want to write about it,
others want to take in this slice of Colorado and enjoy it. Aspen has many other
values such as enhancing wildlife habitat and providing forage for cattle. It also
has an ecological value for forest health and diversity.
Aspen is an unusual tree species. On the one hand, aspen stems are among
the shortest lived in the area and usually die before their first century. On the
other hand, aspen clones are often among the longest-lived individuals in an area,
tenaciously holding to a site and living through many regenerating events, to
maximum ages of centuries or even millennia (Johnston 1996).
Aspen is one of the most shade-intolerant species in the Rockies (Shepperd
1985). Most stands in the Rockies reproduce by root suckering after a disturbance to existing overstory stands (DeByle and Winokur 1985). This is also true
of treated aspen stands throughout the Salida and San Carlos Districts of the San
Isabel National Forests.
USDA Forest Service Proceedings RMRS-P-18. 2001.
1Pike
and San Isabel National Forests,
USDA Forest Service, Pueblo, CO.
377
Benedict
Aspen Regeneration in South-Central Colorado, San Isabel National Forest
Figure 1—Aspen landscape on the San
Isabel National Forest.
Figure 2—Suitable and nonsuitable sites
for aspen harvest on the San Carlos and
Salida Ranger Districts, San Isabel
National Forest.
378
USDA Forest Service Proceedings RMRS-P-18. 2001.
Aspen Regeneration in South-Central Colorado, San Isabel National Forest
Benedict
Regeneration Data
Treated aspen sites were queried from the Region 2 Resource Information
System (R2RIS) and site folders reviewed from 1980 to 1997 for the Salida and
San Carlos Districts, as this is where the majority of the aspen sites reside on the
San Isabel National Forest. These data and associated information are summarized in appendix 1, which lists the most current regeneration survey, stems per
acre, and other key data. The regeneration surveys consisted of walk-throughs
that sampled representative regeneration or systematic sampling of the site with
th
th
fixed plots (1/100 or 1/300 acre), generally 1 plot per acre for the site. For
example, if you had a 20-acre site, crews would sample 20 plots of the site in a
general grid of three chains by three chains.
Salida Ranger District Aspen
Appendix 1 illustrates 13 aspen clearcuts varying in size from 11 to 45 acres.
There were some small patch cuts varying in size from 2 to 5 acres. The aspen
cover types greater than 11 acres seemed to regenerate well. The number of
stems/acre ranged from 800 to 9,000, generally in the fifth year following
harvest (figure 3). The median for stems/acre is 2,400. The small patch cuts
appeared to regenerate (on some patch cuts upward to 2,000 stems/acre from
earlier regeneration surveys); but over time, due to repetitive big game and cattle
damage and other reasons not known, they failed to survive.
The average stocking percentage for the 13 aspen sites clearcut is at 90% of
minimum stocking and distribution. The Forest Plan states a minimum stocking
of 300 trees/acre and 75% of the plots are stocked. However, one of those sites
regenerated mainly into conifer. The elevation ranges are 9,400 to 9,800 feet.
Aspect did not limit regeneration. All aspen types that were clearcut averaged 21
acres. The average heights of aspen 3 to 5 years after harvest ranged from 1 to
5 feet with the majority of sites being 3 feet. There was no site preparation on
any of these sites indicated in the records.
The treatment or disturbance from cutting encouraged the regeneration of
the mature aspen stands that were harvested. When movement of auxin into
roots is halted or reduced by cutting, burning, girdling, or defoliation of trees,
auxin levels in the roots decline rapidly (Eliasson 1971, 1972). This permits new
7
Number of Sites
6
Figure 3—Summarized regenerated
stems/acre for 12 aspen sites harvested
from 1984 to 1991, Salida Ranger District, San Isabel National Forest. (One
stand regenerated to conifer and was
not included in the graph.)
5
4
3
2
1
0
800-1000
1000-2000
2000-4000
4000+
stems/acre
USDA Forest Service Proceedings RMRS-P-18. 2001.
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Benedict
Aspen Regeneration in South-Central Colorado, San Isabel National Forest
suckers to begin; it also allows preexisting primordial, buds, and shoots whose
growth had been suppressed by auxin to initiate growth (DeByle and Winokur
1985).
Aspen Regeneration in Conifer Stands
Following harvest, the Douglas-fir and some Engelmann spruce stands in
the Arkansas Hills area (East Salida District—Kaufman Ridge, Herring and
Bald Mountain) regenerated primarily to aspen (appendix 2). This was found
quite unexpectedly in the Herring Timber Sale area when units planted to
conifers began sprouting aspen. However, in the original planning this was not
apparent. There was an existing aspen root system or remnant aspen scattered
throughout these conifer sites that consisted mainly of Douglas-fir in the
Kaufman Ridge area. Individual genotypes can cover several hectares and ramet
generations may persist over 150 years (Shepperd 1981). The Herring Creek
area was primarily Douglas-fir with a lodgepole pine component on three sites.
When these conifer stands were harvested, these areas sprouted in aspen.
Regeneration surveys showed mainly aspen regeneration with some conifer
reproduction (appendix 3). The stems/acre varied from 855 to 2,550 mainly on
level ground or northern aspects. This was 4 years after harvest. In the Kaufman
Ridge area, aspen stems/acre ranged from 652 to 1,233 with heights of 1 to 3
feet. In 1998, aspen had grown to 6 feet plus.
According to research in Colorado of eight plant species studied, Berndt and
Gibbons (1958) found quaking aspen roots to have the greatest lateral extent,
up to 48 feet from the tree. Also, several studies of soil water depletion by aspen
imply effective rooting depth to be at least 9 feet on deep well-drained soils
(Johnston 1970; Johnston et al. 1969). The scattered aboveground aspen and
the existing root systems were present for quite some time in the Kaufman Ridge
and Herring areas, and the harvest disturbance caused the aspen to sprout.
It is theorized that during past succession cycles, an aspen stand was
ultimately replaced by Douglas-fir in these areas. If a coniferous seed source is
present, young conifers will soon begin to establish themselves under aspen
(Shepperd 1985). The aspen acts as a nurse for the more tolerant conifers
(usually spruce, subalpine fir, or Douglas-fir, but in some cases ponderosa pine
and lodgepole), resulting in a mixed aspen/conifer stand (Shepperd 1985).
Wildland and Prescribed Fire
Possibly in the 1800s, prolonged drought conditions encouraged wildfire
that created sufficient disturbance to conifer stands and as a result aspen
sprouted. Perhaps this succession cycle continued for a long period. Another
possible source of ignition for fires were human caused (i.e., Native Americans
using fire to improve area for big game). Even a mere scattering of aspen in a
coniferous stand commonly will restock the area with a new aspen forest after
a severe wildfire (DeByle and Winokur 1985).
There were 59 fires recorded from 1970 to 1992 on the east side of the Salida
District (figure 4). Thirty-one of these fires were fires less than 1⁄4 acre. These
small fires comprised the majority of fires. The policy during this timeframe was
to suppress fires as soon as they were reported. Most fires were kept small due
to suppression efforts. Typically, unless there is a very dry year associated with
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USDA Forest Service Proceedings RMRS-P-18. 2001.
Aspen Regeneration in South-Central Colorado, San Isabel National Forest
Benedict
35
Number of Fires
30
25
Figure 4—Fire frequency from 1970 to
1992 for the East Side of Salida District,
San Isabel National Forest.
20
15
10
5
0
< 1/4 ac
1/4 to 10 acs
10 to 100 acs
Fire Size
high winds, most fires stay small. The typical fire season is from mid-April to
early/mid July. Most of the lightning in this time frame is late June to early July
followed by monsoon rains around the Fourth of July.
Lightning was the main cause (83%) with the remainder human caused
(17%)(table 1). One could surmise that lightning-caused fires in the 1800s
(prior to suppression efforts) along with other causes that created disturbance
quite possibly regenerated aspen.
There were only six of the 59 fires between 10 and 100 acres. Although this
is not a long period, it does indicate that in a 22-year timeframe fires have not
grown to large size. This would show a minimal disturbance from wildland fire.
This could be attributed to the aggressive program to put out fires, lack of fuel
continuity, and green vegetation.
The Bald Mountain Prescribed Burn also regenerated aspen (appendix 2).
This was observed 2 to 3 years after treatment in an aspen patch that had
sprouted and grown very dense. Most prescribed burns in the aspen type are
designed to regenerate declining, overmature clones. To maximize sprouting, at
least 80% of the overstory should be killed (Powell 1988). Research indicates
that fires with flame lengths of 1.7 to 2.1 feet are required to kill aspen trees. This
equates to fireline intensities of 18 to 28 BTU/ft/sec (Brown and Simmerman
1986). To achieve this intensity in aspen community types with undergrowths
dominated by shrubs or tall forbs, the herbaceous vegetation should be at least
50% cured (Brown and Simmerman 1986). A good burning window to create
a high burning intensity in aspen sites is immediately after leaves have fallen in
September and before leaves are compacted.
Table 1—Fire cause summary (1970–1992, east
side Salida District).
Cause
Lightning
Campfires
Smoking
Fire occurrence
48
8
2
Percentage
83
14
3
Source: GIS fire history maps.
USDA Forest Service Proceedings RMRS-P-18. 2001.
381
Benedict
Aspen Regeneration in South-Central Colorado, San Isabel National Forest
San Carlos District Aspen
The San Carlos District has aspen intermixed in the conifer stands throughout the district. In one instance a unit in the Little Froze Timber Sale (located
in the Wet Mountains) regenerated 500+ stems/acre on a conifer site after being
harvested. Aspen sites throughout the district are experiencing a conifer
invasion. Generally, there has been a lack of disturbance. From the R2RIS
records, only 172 acres of aspen have been treated since 1986. It has been
observed that aspen encroachment is occurring in natural meadows as aspen
crowd the edge and tiers of different age classes (Mike Smith, personal
communication).
The Forest Plan (Land and Resource Management Plan, Pike and San Isabel
National Forests, Comanche and Cimarron National Grasslands) states that
wildfire has historically been the primary disturbance initiating aspen root
sprouting. Control of wildfire has permitted many aspen stands to become
overmature with little success in regenerating. In the absence of disturbance,
either natural or human caused, much of the aspen will convert to conifer types
in 100 to 200 years.
Management Recommendations
It is clear that to sustain healthy aspen on the San Isabel National Forest,
disturbance/management must take place to regenerate the aspen component
and reduce the conifer invasion. Here are some recommendations:
382
•
Manage aspen sites at a minimum of 8 to 10+ acres. Small patches seem
to be vulnerable to big game or cattle damage. Consider treatments
scattered over a large landscape.
•
In areas where access is limited or does not exist, consider prescribed
burning to meet resource goals and objectives. It is critical to burn at an
intensity to kill a majority of the overstory to obtain regeneration.
•
Explore market opportunities to encourage future aspen treatment.
•
Aspen management does not need site preparation to regenerate aspen.
This will serve as a reduced cost when considering other conifer species
that will require site preparation.
•
Consider conifer sites (i.e., Douglas-fir) with an aspen remnant or root
system that will sprout/regenerate after disturbance (prescribed fire or
cutting). Some indicators of aspen presence are: downed aspen from
past years, occasional live standing aspen, and adjacent aspen stands.
•
Consider wildland fire use and integrated fire planning through an
approved Fire Management Plan. This examines the appropriate management responses to a management area. In some areas “fire breaks” of
pure aspen are being considered so that a catastrophic fire could stopped
much easier by running into pure aspen.
USDA Forest Service Proceedings RMRS-P-18. 2001.
Aspen Regeneration in South-Central Colorado, San Isabel National Forest
•
Benedict
Look at boundaryless management through agreements and memorandums of understanding between state and federal agencies to treat the
landscape.
Aspen sites throughout the San Isabel National Forest need further management due to lack of disturbance. Treatment and care of this important species
is critical to continued scenic beauty, forest health/diversity, wildlife, range and
forest management, and sustainability of our forest for future generations.
References
Berndt, Herbert W. and Robert D. Gibbons. 1958. Root distribution of some native trees and
understory plants growing on three sites within ponderosa pine watersheds in Colorado.
Station Paper 37, U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest
and Range Experiment Station. 14 p.
Brown, James K. and Dennis G. Simmerman. 1986. Appraising fuels and flammability in western
aspen: A prescribed fire guide. Gen Tech. Rep. INT-205, Ogden, UT: U.S. Department of
Agriculture, Forest Service, Intermountain Research Station. 48 p.
DeByle, Norbert V. and Robert P. Winokur, editors. 1985. Aspen: ecology and management in
the western United States. Gen. Tech. Rep. RM-119, Fort Collins, CO: U.S. Department
of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station.
Eliasson, L. 1971. Growth regulators in Populus tremula: 4. Apical dominance and suckering in
young plants. Physiologia Plantarum 25:263–267.
Eliasson, L. 1972. Translocation of shoot applied indolylacetic acid in roots of Populus tremula,
Physiologia Plantarum 27:412–416.
Johnston, Robert S. 1969. Aspen sprout production and water use. Res. Note INT-89, Ogden,
UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range
Experiment Station. 6 p.
Johnston, Robert S. 1970. Evapotranspiration from bare, herbaceous, and aspen plots: a check
on a former study. Water Resources Research 6(1):324–327.
Johnston, Robert S. 1971. Rainfall interception in a dense Utah aspen clone. Res. Note INT-143,
Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and
Range Experiment Station. 9 p.
Johnston, Berry C. 2000. Report, regeneration of aspen clearcuts of the Uncompahgre Plateau,
Colorado. U.S. Department of Agriculture, Forest Service. 3 p.
Powell, David C. 1988. Aspen community types of the Pike and San Isabel National Forest in
South-Central Colorado. R2-ECOL-88-1: 28, 35–147.
Shepperd, Wayne D. 1981. Stand characteristics of Rocky Mountain aspen. In De Byle, N.V.,
ed. Situation management of two intermountain species: aspen and coyotes, Symposium
proceedings, Volume I, Aspen; 1981 April 23–24. Logan, UT: Utah State University:
22–30.
Shepperd, Wayne. 1985. Aspen ecology and management in the Central and Southern Rocky
Mountains, Proceedings of a symposium; 1985 July 28–31, Fort Collins, CO. Society of
American Foresters, National Convention: 233–234.
Smith, Michael. 2000. Personal communication. Silviculturist, U.S. Department of Agriculture,
Forest Service, San Carlos Ranger District, Canon City, CO.
USDA Forest Service Proceedings RMRS-P-18. 2001.
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Aspen Regeneration in South-Central Colorado, San Isabel National Forest
Appendix 1: Regeneration Surveys of Treated Aspen Sites of the
Salida District, San Isabel National Forest
Location
and site
Regen.
survey
type and
1
year
Acres
Elevation
Aspect
Harvest
year
101301004
22
—
ft.
9,700
NE
1987
1996
P
2,086
100%
3 ft.
101301014
45
—
9,900
NE
1984
1989
W
3,000–4,000
95%
4–5 ft.
101301036
42
—
9,900
NE
1985–1986
1988
P
2,500+
100%
3–4 ft.
101301043
17
—
9,700
NE
1986
1989
W
2,500+
100%
3.5 ft.
1013001044
22
—
9,700
NE
1987
1992
W
4,000
90%
3–5 ft.
1014050100
16
POTRFestuca
9,800
N
1988
1992
W
2,400
50%
5 ft.
POTRFestuca
9.800
Level
1989
1994
W
9,100
100%
POTRFestuca
9,800
Level
1990
1993
P
1,782
100%
0.5 to 2 ft.
POTRFestuca
9,800
Level
1991
1996
P
1,021
100%
3 ft.
—
9,400
Level
1978–1980
1992
W
on the edge
0
Some
aspen regen.
0%*
—
9,800
Level
1980–1981
0
Some
0%*
for
stocked site
1992
W and P
aspen regen.
on the edge
Level
Level
and
1993, 5 acs
Approx.
1980, 4 acs
1996
W
1992
W
1,000
Mainly conifer
0
0%*
Thurberi
1014050102
1014050105
1014050106
101801003
(11 acs)
16
Thurberi
11
Thurberi
19
Thurberi
4 (1–5 acre)
patch cuts
101806008
(13 acs)
5 (1–4 acre)
patch cuts
101806012
4 and 5 acre
patch cuts(9)
102311066
14
# of
stems/acre
% stocked
and ave.
height
Plant
assoc.
patchy
—
9,600
9,500
100%
POTRFestuca
9,500
Level
1989
1996
P
833
87%
2 ft.
Thurberi
10220646
29
—
9,400
S
1983–1984
1989
P
3,300
88%
102206047
22
—
9,400
S
1986
1990
800
100%
102206048
11
—
9,500
E
1987
1992
1,264
63%
1
P = Systematic sampling with plots and W = Walk-through.
*Big game and cattle damage, 101801-003 and 101806-008 showed earlier surveys with regeneration.
384
USDA Forest Service Proceedings RMRS-P-18. 2001.
Aspen Regeneration in South-Central Colorado, San Isabel National Forest
Benedict
Appendix 2: General Locations of Aspen
Regeneration in Conifer Stands
USDA Forest Service Proceedings RMRS-P-18. 2001.
385
Benedict
Aspen Regeneration in South-Central Colorado, San Isabel National Forest
Appendix 3: Aspen Regeneration in Conifer Stands (Mainly
Douglas-fir) of the East Salida Ranger District, San Isabel National
Forest
Location
and site
Acres
Plant
assoc.
Elevation
Aspect
Harvestyear
1
and method
Regen.
survey
type and
1
year
# of
2
stems/acre
% stocked
and ave.
height
ft.
Herring Creek
101405107
1014050108
101405109
Kaufman Ridge
1014020097
1013030067
101305086
Beaver Creek
102311069
102311070
9
26
15
PSME/ARUV
JUCO
PSME/ARUV
JUCO
PSME/ARUV
JUCO
9,880
Level to E
9,880
NE
9,880
N-NE
38
PSME/JAAM
9,850
NE
23
PSME/JUCO
9,800
NE
28
PSME/ARAD
JUCO
9,700
NE
5
PIPU-PSME
JUCO
PIEN/JUCO
PICO/JUCO
9,100
NE
9,360
NE
43
1990
CC
1990
ST
1990
CC
1996
1996
1996
1992
ST
1992
ST
1992
ST
1996
1993
CC
1993
ST
1996
1996
1996
1996
855
P
2,550
P
1,353
89%
2 ft.
100%
3 ft.
93%
1,137
P
652
P
1,233
P
89%
1–3 ft.
91%
3 ft.
92%
2–3 ft.
350
P
440
P
100%
91%
1–3 ft.
Note: The majority of the regeneration is aspen with some conifer intermixed.
1
ST = Seed tree and CC = Clearcut.
2
P = Systematic sampling with plots.
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USDA Forest Service Proceedings RMRS-P-18. 2001.